- move _uniqueChild earlier since a comment now mentions it earlier.
- reorder methods in Element to more closely reflect call order.
- change mount to be the place that sets the parent pointer, for consistency.
- make the lifecycleState a purely debug-time thing for consistency.
- make BuildableElement.unmount set dirty to false so that we won't
  build unmounted objects.
- rename "updated" to "newWidget" for clarity.
- change how we unmount things so that the slot is reset up to a
  RenderObjectElement, but not further, and don't reset the depth.